This invention relates to switching power converters of the push-pull type, and more particularly to an improved snubber circuit for safeguarding the switching transistors against peak stresses.
In a conventional push-pull power converter, the source of DC power is alternately connected to two sides of a center tapped primary winding in a transformer by two control transistors periodically switched on 180.degree. out of phase. In a converter which incorporates output voltage regulation, the period that each control transistor is turned on is regulated through a pulse width modulator as a function of the output voltage. U.S. Pat. No. 3,670,234 discloses such a pulse width modulated voltage regulator for a driven power converter. Each of the control transistors is protected by a diode which conducts current in a direction opposite to the current through the control transistors to protect them against reverse current when they are turned off due to energy stored in the primary winding of the transformer. Such a technique for protecting a switching transistor having an inductive load has become quite conventional.
One of the problems encountered in the operation of such a push-pull regulating converter is that the presence of an inductive load on the switching transistors produces power peaking when the transistor that is conducting is switched off. This could result in overstressing the switching transistors. FIG. 1 illustrates a basic push-pull converter, and FIG. 2 illustrates the peak power of the switching transistor Q.sub.1 or Q.sub.2 which is the product of its collector current, I.sub.C and its collector-emitter voltage V.sub.CE, at turn-off. This peaking results because the voltage across the off-going transistor must reach the level of the input voltage before it can be turned off and the load current begins to flow independent of the transistor being switched off.
In practical circuits, the transformer T.sub.1 causes the collector-emitter voltage to be twice the level of the input voltage E.sub.IN. This high voltage, 2E.sub.IN, and collector current, I.sub.C, produce in the transistor being switched off a peak power point which may damage, or decrease the life of, the transistor. This undesirable situation has been handled by use of a "snubber" circuit consisting of a resistor or a capacitor as shown in respective U.S. Pat. Nos. 3,430,125 and 3,305,756, or with both a resistor R.sub.1 and a capacitor C.sub.1 as shown in FIG. 3. The curve shown in FIG. 4 indicates the change in parameters resulting from this snubber circuit. Peaking and the total energy which must be absorbed by the switching transistor are reduced. A disadvantage is that power is wasted in the snubber circuit. A better prior-art snubber circuit is shown in FIG. 5 in which the circulating load current is diverted through diodes D.sub.1 and D.sub.2 shunting resistors R.sub.1a and R.sub.1b to charge capacitors C.sub.1a and C.sub.2b as the voltage rises across the transistor during turn-off. Each diode provides a low impedance current path during turn-off and forces current through the current-limiting resistor during turn-on. This results in the somewhat better curve shown in FIG. 6. A disadvantage of these snubber circuits shown in FIG. 3 and FIG. 5 is that both require use of low dissipation type capacitors and resistors having fairly large power ratings, with that of FIG. 3 having a slight advantage in that fewer components are required. Both do reduce considerably the power dissipated in the transistors, at a small penalty of somewhat slower switching capability, but both waste power dissipated in the resistors. In addition, the wasted power produces heat which imposes an undesirable thermal load that must be taken care of, perhaps at some further cost in energy consumption. It would be desirable to provide a snubber circuit in which energy absorbed by the circuit is returned to the power supply instead of being dissipated in the circuit. That would both reduce thermal load and conserve energy in the power supply.
The energy dissipated by the prior art snubber circuits can be determined by calculating the energy transfer from the snubber capacitor during each switching cycle as 1/2CV.sup.2, and multiplying by the frequency of operation, f, as follows: EQU P.sub.avg =fC(V.sup.2 /2)
Applying this equation to a power supply system in which four 400 watt power supplies are paralleled, each having an input varying from 200 to 400 volts and an operating frequency of 10 kHz, and in which transistor collector current I.sub.c is 5 amperes, with a minimum operating voltage of 200, a changing voltage .DELTA.t=0.5.times.10.sup.-6, the value of C becomes EQU C=(I.sub.c .DELTA.t/.DELTA.V)mfd
Substituting values: ##EQU1## and EQU C=0.0125mfd
The average power dissipated in each side (phase) by the resistor is: EQU P.sub.avg =fC(V.sup.2 /2) EQU P.sub.avg =(10.sup.4) (0.0125.times.10.sup.-6) (400).sup.2 /2 EQU P.sub.avg =10 watts
The loss of 20 watts per module for the four modules amounts to 80 watts. While this is only 5% of the total of 1600 watts, the heat load must be thermally accounted for and rejected. Any improvement which minimizes the thermal load and increases the efficiency of the system by recovering and conserving much of this wasted energy is therefore of interest.